The present invention relates to a device for providing a focus signal at the position of focus of a photographing lens on an object to be photographed in a device which automatically detects focus by detecting the contrast of the image.
It is a fundamental property that the contrast of light and shade of the optical image of an object to be photographed by a photographing lens is maximized when the image of the object is focused. This is due to the fact that the power spectrum (optical intensity) of each spatial frequency of the image of the object becomes a maximum at the point of focus. A variety of automatic focus detecting devices have been proposed which utilize this phenomenon.
Examples of such devices are generally simple in construction. More specifically, in such a device, a contrast detecting photoelectric element is disposed in a plane which is optically equivalent to the surface of a film and a contrast signal is extracted from each object image projecting onto the photoelectric element. The contrast signal thus produced becomes a maximum at the point of focus of the photographing lens. That is, the characteristic curve of amplitudes of the contrast signal as the photographing lens is moved over its range has a single peak. Accordingly, in order for the device to detect the focus position, it is necessary to provide some means for detecting the maximum value of the contrast signal. One example of such means is a measuring instrument with a pointer which is so designed that the deflection of the pointer is in proportion to the magnitude of the contrast signal. With this measuring instrument, the photographer can visually detect the point of maximum contrast from the maximum deflection point of the pointer. However, it is often rather troublesome and difficult for the photographer to detect the maximum deflection of the pointer. If it is possible to produce a focus signal in such a manner that a lamp is turned on with the aid of the focus signal only at the position of focus, then the photographer can readily determine that the photographing lens is focused on an object to be photographed. However, it should be noted that it is difficult to produce such a focalization signal from only a single contrast signal as described above because different contrasts are obtained from different objects within the view of the photographing lens and accordingly the contrast signal produced at the point of focus is not always constant or the signal peak corresponding to the desired object.
Production of a focalization signal from a single contrast signal as described above may be achieved by a device in which the maximum value of a contrast signal is stored as the lens moves in one direction. The stored value and the present contrast signal as the lens is moved reciprocatingly are compared and the focus signal is generated when the two are equal. However, the device suffers from two serious problems. One of the two problems is that, if the photographing lens is moved rearwardly before it reaches the true focus position, the contrast signal shows only an apparent maximum value. That is, a false peak signal is provided in response to which a false focus signal is produced. The other problem is that, before the photographing lens is focused on a new object, it is necessary to reset or clear the contents which have been stored for the previous object.
Because of the above-described difficulties, totally effective focus signals cannot be provided by such conventional devices which simply produce a contrast signal from the image of an object. In order to overcome these difficulties, a variety of improved techniques have been proposed in the art. In one such technique two contrast signals provided by two contrast detecting photoelectric elements which are spaced apart from each other in the direction of the optical axis are compared to obtain a focus signal. In another such device, a single contrast detecting photoelectric element is vibrated in the direction of the optical axis to obtain two contrast signals which are compared to obtain a focus signal similar to the above-described device. However, in such a device in which the contrast signals of two object images spaced in the direction of the optical axis are extracted spatially or with a time division technique are generally large in scale and high in manufacturing cost.
The aforementioned focus signal is a focus indicating signal which is produced in a digital mode at the position of focus. However, it should be noted that the focus indicating signal must be very high in accuracy to be acceptable commercially. Especially with a high grade camera such as a single-lens reflex camera, sharp pictures cannot be obtained without an indicating focus device having a very high accuracy. When pictures are taken with a camera held by hand, the camera is liable to be shaken making it rather difficult to obtain sharp pictures. If, in this case, the focus signal has an extremely high focus indication accuracy, the display of the focus signal may repeatedly turn on and off making the camera difficult for the photographer to use. Accordingly, the focus signal should have a high focus indication accuracy for taking pictures with a camera which is fixed in position. However, the accuracy should not be so high for taking pictures when the camera is held by hand.
Accordingly, an object of the present invention is to provide an automatic focus indicating device in which all of the above-described difficulties have been eliminated and in which the focus signal allowing the light emitting element to operate at the position of focus is produced by extracting a single contrast signal from the image of an object to be photographed. The focus signal should be high in focus indication accuracy for taking pictures when the camera is fixed in position but not so high in focus indication accuracy for taking pictures with the camera held by hand. The device according to the invention is meritorious and effective in that it is compact and low in manufacturing cost and the focus position can be readily determined for taking pictures with a camera held by hand and, furthermore, the focus indication accuracy is considerably high for taking pictures with the camera fixed in position.